1. Field of the Invention
The present invention relates to compositions of matter which promote the escape of air, water, or other volatile materials from curable film-forming resins before or during the resin curing process. More specifically, the invention relates to degassing agents and degassing compositions for curable coating compositions.
2. Description of the Related Art
Coatings which are intended to cure in place and produce a relatively smooth or transparent surface are naturally susceptible to defects stemming from entrapped bubbles. Defects associated with entrapped bubbles are known in the coatings industry as pinholing, cratering, cracking, orange peel, and loss of gloss, for example, and sometimes indicate diminished chemical and physical properties in the affected portion of the coating. The coatings may be solvent-based coatings, water-based coatings or powder coatings, to name a few.
Bubble formation is typically related to a change in physical conditions such as heating or cooling, or a chemical reaction within the coating. Many traditional curing processes for coatings involve chemical reactions which routinely produce bubbles containing air, water, nitrogen, or volatiles such as evaporated solvent, among other things. If the material within the bubbles escapes before the coating hardens, any possible defect in the coating is minimized or avoided. Degassing agents have been developed over the years to promote the timely escape of material from the bubbles into or through the coating.
U.S. Pat. No. 3,872,040, issued to Mollohan et al., describes powder coating compositions formulated to contain a minor proportion of a natural or synthetic wax. The wax is said to promote films having smooth surfaces which are substantially free of voids or entrapped gas pockets.
U.S. Pat. No. 5,153,252, issued to Skora, describes a melt viscosity modifier which reportedly acts to permit the release of entrapped gases and volatiles from a thermosetting powder coating film and, thereby, prevent the formation of pin-holes. The Skora patent states that the melt viscosity modifier has the structure of bis-phenoxy propanol and is preferably 1,3-bis-phenoxy-2-propanol.
U.S. Pat. No. 5,342,893 issued to Wilkinson, et al., describes a thermosetting powder coating composition containing, as an anti-popping agent, a thermoplastic polymer which i) is solid, ii) has a Tg of 30° C., iii) comprises, at least mainly, units derived from at least one alkyl acrylate or alkacrylate and iv) has an acid value of no more than 10.
A European Patent Application published under number EP 0 471 409 reports a degassing agent for powder coatings which is a crystalline wax having a low surface tension, a melting point in the range of 100 to 160° C., and a viscosity measured 10° C. above the melting point of less than 50 mPas. The '409 application also describes the degassing agent as containing an amide bond and a fatty acid group, such as the distearyl amide of ethylene diamine.
U.S. Pat. No. 6,100,349, issued to Koenraadt et al., describes benzoin and cyclohexane dimethanol dibenzoate as degassing media suitable for use in a thermosetting powder coating composition. While benzoin is widely used in the coatings industry, it has been known to discolor white and light-colored coatings, and to cloud transparent and translucent coatings under some circumstances.
U.S. Pat. No. 6,184,279, issued to Anderson et al., describes compositions including the cured coatings of at least one curable film-forming resinous material along with an effective degassing amount of a material having the structure:
and dimer derivatives thereof wherein R and R′ are the same or different and are primary alkyl groups with 1 to 6 carbon atoms including methyl or an aromatic group containing at least one heteroatom.
Volatile organic compounds (VOC) are suspected of depleting of the ozone layer, which shields humans from dangerously high levels of ultraviolet radiation (UVR). Significantly depletion of the ozone layer would likely result in higher incidence of skin cancer and related ailments. While VOC is known to emanate from a variety of sources, solvent based coatings and water based coatings are generally considered to be contributing to the problem.
The coatings industry is working on several fronts to lower VOC content in coatings. One major effort involves switching from organic solvent based coatings to water based coatings, which contain relatively less VOC. Also, efforts are being made to completely avoid solvents by formulating more and better powder coatings and UV curable coatings. Powder coatings are applied as solids at room temperature and, subsequently, cured by heating to a liquid form in which cross linking chemical reactions cure and harden the coating. While the coating is in liquid form, additives such as flow agents, leveling agents and degassing agents play important roles in keeping the coating smooth and bubble-free.
For many years, benzoin has been the additive of choice for the prevention of pinholes primarily due to its relatively high efficiency and low cost. Benzoin does, however, have drawbacks. One drawback is its tendency to impart yellowness to coatings, especially white or clear coatings. A second drawback is that benzoin sublimes at elevated temperature and tends to coat the bulbs of infra-red lamps which are commonly used for curing the coatings. A third drawback is that benzoin decomposes and discolors over time, particularly in the presence of ultra-violet radiation and at elevated temperature.
Much effort has been devoted to developing better degassing agents, and the accomplishments of previous researchers are numerous and impressive. However, a need still exists for an improved degassing agent which is affordable and effective, will not discolor white and light-colored coatings, and will not cloud transparent and translucent coatings.